This paper aims at studying the combination of different collaboration modes between operator and collaborative robot in order to optimize an assembly process for both economic and ergonomic objectives. Based on a real case study, and using a energy expenditure ergonomic model, the authors have determined by experiment the different ergonomic and economic variables under each possible collaboration mode. They propose a set of indicators to evaluate the quality of assignment solutions, as well as a multi-objective cost function to determine optimal trade-offs between the different collaboration modes. An initial set of trials has indicated that combining several modes of collaboration may deliver benefits for both economic and ergonomic performance.